feat: add optimized field load/store instructions for direct method access

Introduce CODE_LOAD_FIELD_THIS and CODE_STORE_FIELD_THIS opcodes that bypass receiver stack operations when field access occurs directly within a method body. Refactor field() compiler to emit these faster instructions when compiler->methodName is set, and extract loadThis() helper to correctly resolve 'this' in nested functions. Update VM interpreter with dedicated dispatch cases that read receiver from frame->stackStart instead of popping, and add debug disassembly support. Add test cases for field closure capture and nested class field isolation, removing related TODO comments.

test/field/multiple.wren

@@ -6,6 +6,7 @@ class Foo {
     _d = d
     _e = e
   }
+
   write {
     IO.write(_a)
     IO.write(_b)
@@ -23,7 +24,3 @@ foo.write
 // expect: 3
 // expect: 4
 // expect: 5
-
-// TODO: Trying to get or set a field outside of a class.
-// TODO: Fields in nested classes.
-// TODO: Closing over fields.